Stabilized Synthesis of 2D Verbeekite: Monoclinic PdSe Crystals with High Mobility and In-Plane Optical and Electrical Anisotropy.

PdSe has a layered structure with an unusual, puckered Cairo pentagonal tiling. Its atomic bond configuration features planar 4-fold-coordinated Pd atoms and intralayer Se-Se bonds that enable polymorphic phases with distinct electronic and quantum properties, especially when atomically thin. PdSe is conventionally orthorhombic, and direct synthesis of its metastable polymorphic phases is still a challenge. Here, we report an ambient-pressure chemical vapor deposition approach to synthesize metastable monoclinic PdSe. Monoclinic PdSe is shown to be synthesized selectively under Se-deficient conditions that induce Se vacancies. These defects are shown by first-principles density functional theory calculations to reduce the free energy of the metastable monoclinic phase, thereby stabilizing it during synthesis. The structure and composition of the monoclinic PdSe crystals are identified and characterized by scanning transmission electron microscopy imaging, convergent beam electron diffraction, and electron energy loss spectroscopy. Polarized Raman spectroscopy of the monoclinic PdSe flakes reveals their strong in-plane optical anisotropy. Electrical transport measurements show that the monoclinic PdSe exhibits n-type charge carrier conduction with electron mobilities up to ∼298 cm V s and a strong in-plane electron mobility anisotropy of ∼1.9. The defect-mediated growth pathway identified in this work is promising for phase-selective direct synthesis of other 2D transition metal dichalcogenides.